Why Europe went looking for a sea route 

Constantinople did not sit on the spice trade so much as beside it. Pepper and other eastern goods reached Europe mainly through Alexandria and the Levant, in Venetian and Genoese hulls, under the taxation of whoever held Egypt and Syria. What the Ottomans acquired in 1453, and then extended over the following seventy years, was control of the whole crossroads: the Black Sea grain and slave trade, the overland routes through Anatolia, and eventually the Mamluk lands themselves after 1517.

The effect was not a wall across Asia. Trade continued, because taxing trade is more profitable than stopping it, and Venice negotiated its way into a working relationship with the sultans within a few years. The effect was on cost and control. Every additional customs post and every additional intermediary raised the landed price of eastern goods in Europe, and the profit accumulated in the hands of the Ottomans and the Venetians who dealt with them. For the Atlantic kingdoms, particularly Portugal and Castile, that meant paying a premium set by rivals.

Portugal's response predates 1453 and is the honest reason this chapter is careful about causation. Prince Henry's captains had been pushing down the African coast since the 1420s, hunting gold, fish, slaves and a way around the Saharan caravan trade. The fall of Constantinople did not launch that programme, but it made the case for it unanswerable and it removed any expectation that the eastern approaches would become cheaper. Progress after mid-century is rapid. Cape of Good Hope rounded by Bartolomeu Dias in 1488. Columbus sailing west under Castilian contract in 1492. Vasco da Gama reaching Calicut in 1498, and Portuguese ships buying pepper at source within a decade after that.

The result was a slow reversal of Europe's economic geography. The Mediterranean had been the centre and the Atlantic seaboard the edge. Once cargo could travel from India to Lisbon without a single overland toll, the edge became the centre, and Venice and Genoa began the long decline from world powers to regional ones. The Ottomans held the old crossroads for centuries more, but the traffic had found another road.

That is what makes 1453 a genuine hinge rather than a famous date. In one morning it ended the Roman state, confirmed the fastest-growing empire in the world, dispersed the libraries of the Greek East into a Europe that had just invented printing, and made an ocean look like a cheaper option than a road. Nobody standing at the Kerkoporta could have described any of that, which is generally true of the days that turn out to matter.

The Middle Ages did not end at a stroke, of course. But if you were looking for the morning to point at, this is the one historians keep coming back to.